The present invention pertains to methods for stripping photo-resist material and removing etch-related residues from the surface of a partially fabricated integrated circuit in preparation for further processing. More specifically, the invention pertains to methods for implementing a plasma operation using hydrogen and a weak oxidizing agent such as carbon dioxide. The invention is effective at efficiently stripping photo-resist and removing residues from low-k dielectric layers after etching processes used in Damascene devices.
Damascene processing techniques are often preferred methods in many modern integrated circuit manufacturing schemes because it requires fewer processing steps and offers a higher yield than other methods. Damascene processing involves forming metal conductors on integrated circuits by forming inlaid metal lines in trenches and vias in a dielectric layer (inter-metal dielectric). As part of the Damascene process, a layer of photoresist is deposited on a dielectric layer. The photoresist is a light-sensitive organic polymer which can be “spun on” in liquid form and dries to a solid thin film. The photosensitive photoresist is then patterned using light through the mask and wet solvent. A plasma etching process (dry etch) is then used to etch exposed portions of dielectric and transfer the pattern into the dielectric, forming vias and trenches in the dielectric layer.
Once the dielectric layer is etched, the photoresist must be stripped and any etch-related residues must be thoroughly removed before subsequent processing to avoid embedding impurities in the device. Conventional processes for stripping photoresist employ a plasma formed from a mixture of gases with the presence of oxygen in the plasma. The highly reactive oxygen based plasma reacts with and oxidizes the organic photoresist to form volatile components that are carried away from the wafer surface.
Highly oxidizing conditions are also generally unsuitable for use on low dielectric constant (low-k) materials. Low-k materials have been used as inter-metal and/or inter-layer dielectrics between conductive interconnects in many modern devices to reduce the delay in signal propagation due to capacitive effects. The lower the dielectric constant of the dielectric material, the lower the capacitance of the dielectric and the lower the RC delay of the integrated circuit. Typically, low-k dielectrics are silicon-oxide based materials with some amount of incorporated carbon, commonly referred to as carbon doped oxide (CDO). It is believed, although not necessarily proven, that the oxygen scavenges or removes carbon from the low-k materials. In many of these materials such as CDOs, the presence of carbon is instrumental in providing a low dielectric constant. Hence, to the extent that the oxygen removes carbon from these materials, it effectively increases the dielectric constant. As processes used to fabricate integrated circuits moves toward smaller and smaller dimensions and requires the use of dielectric materials having lower and lower dielectric constants, it has been found that the conventional strip plasma conditions are not suitable.
To solve these issues, others have reported using hydrogen based plasmas with inert gases or alternative strong oxidizing agents to strip the photoresist. Han et al (U.S. Pat. Nos. 6,281,135 and 6,638,875) describe using a mixture of hydrogen, helium and fluorine and Zhao et al (U.S. Pat. Nos. 5,660,682 and 6,204,192) describe using a mixture of hydrogen and argon. The hydrogen based plasmas without oxidizing agents, however, have very low strip rates and may not effectively remove the myriad of organic contaminants found on the wafer after etching.
What is needed therefore are improved and more efficient methods for stripping photoresist and etch-related materials from dielectric materials, especially from low-k dielectric materials.